The present invention is related to systems for detecting fluorescing particles in a fluid stream, for example as disclosed in U.S. Pat. No. 5,999,250 (Hairston, et al.), incorporated by reference herein. These systems involve directing focused beams of coherent energy onto an aerosol stream at various points along the stream. In particular, two red or near infrared beams in a continuous wave (CW) mode intersect the stream at spaced-apart points, impinging upon particles to generate time-of-flight measurements. An ultraviolet (UV) excitation beam is directed onto the particles downstream, to trigger fluorescence or other responsive emissions by the particles. The UV beam is operated in an on/off mode, triggered to irradiate a particle based on the time-of-flight signal generated by that particle when passing through the longer wavelength beams. Thus, time-of-flight measurements are used both to aerodynamically size the particles and to time each firing of the UV laser.
The present invention is directed to a variety of improvements in these systems, either to simplify the approach and reduce cost yet provide the same level of performance, or to enhance system performance by increasing detection sensitivity, enhancing component life, providing more complete information concerning particle composition, or to eliminate a potential source of time-of-flight measurement uncertainty.
The tendency of biological materials to emit fluorescence energy in response to irradiation by shorter wavelength energy, particularly in the violet and UV ranges, has been usefully employed in single particle analysis systems to detect the presence of biological agents in aerosols. Although specific biological materials differ from one another as to the most effective (peak) excitation wavelength, and as to the wavelengths of emitted fluorescent energy, it remains difficult to differentiate microbiological substances from oils, greases, volatile organic compounds and other ambient background particles, and more so to distinguish biological components from one another. Although this difficulty can be countered by combining other analysis techniques (e.g. mass spectrometry) with fluorescence detection, such combinations can present their own problems. For example, the ablation and ionizing laser in a time-of-flight mass spectrometer requires time between successive firings, to the point of reducing the speed of the overall system. In some situations, the reduced speed may be no more than an inconvenience. In systems designed to detect potentially harmful biological agents in a building ventilation system, the reduced rate may have a critical negative impact.
A concern applicable to all single particle analysis systems is the need for increasing the life of system components, especially the diode lasers and other radiant energy sources. In smaller, more compact systems, a further need arises to overcome the tendency of proximate radiation sources, and the energy scattered or emitted by particles exposed to these sources, from interfering with one another. Finally, there is a need to address the foregoing problems, and at the same time reduce system cost and complexity.